This invention relates to a steering system, and in particular to a steering system intended for use in a downhole environment in the formation of a borehole.
Steering systems for use in downhole environments typically fall into two groups. In one type of steering system, a downhole motor and drill bit are secured to the remainder of the downhole assembly and drill string through an angled component so that the axis of rotation of the drill bit is angled relative to the axis of the adjacent part of the borehole. In normal use, the drill string rotates with the result that the drilling direction constantly changes, the net result of which is to form a generally straight or spiraling borehole. However, if it is desired to drill a dog leg or curve, the drill string is held against rotation with the drill bit pointing in the direction in which the curve is to be formed. After formation of the curve, rotation of the drill string at a generally uniform speed re-commences so as to drill a generally straight borehole region.
In the second type of system, a downhole bias unit is provided to allow the application of a biasing side load to the drill bit to form a curve in the borehole. The bias unit typically comprises a housing on which a plurality of bias pads are mounted, each pad being moveable between retracted and extended positions by respective actuators. By appropriate control of which pad or pads are in engagement with the wall of the borehole being drilled at any given time, the direction in which the biasing side load is applied can be controlled.